The invention relates to the control of the operation of a Diesel engine. More particularly, the invention relates to a method and an apparatus for limiting the maximum fuel quantity which is supplied to a Diesel engine, in particular a super-charged or turbo-charged Diesel engine. The method and apparatus of the invention still more particularly relates to a Diesel engine equipped with a fuel supply pump which supplies fuel as a function of engine variables including the engine speed and at least one other operational engine variable.
Known in the art is a Diesel engine control system which includes a speed governor and a control cam which the speed governor rotates as a function of engine speed. The control cam has a contour which serves to limit the maximum amount of fuel fed to the engine for each particular rpm and takes into account engine characteristics. The control curve of the cam is so designed as to reduce the amount of fuel for a full engine load when the engine speed is decreasing, which prevents an excessive amount of fuel to be supplied to the engine for decreasing rpm and thereby prevents exceeding the permissible smoke limit. This type of so-called negative adaptation is also used in super-charged engines to adapt the operation to account for the increasing air mass which accompanies increasing rpm and increasing induction tube turbo-charged pressure. However, these known methods for adapting the amount of fuel on the basis of rpm alone can take into account only a particular state of operation, for example only the full-load operation defined by a single full-load curve in which the maximum amount of fuel corresponding to a particular rpm and for example to the maximum air mass is limited. In this way, the engine is able to accelerate very rapidly but, when it is being accelerated from the partial-load domain in which the induction tube charging pressure has not yet reached the value corresponding to the instantaneous engine speed, the engine receives an excessive amount of fuel which causes the exhaust gases to exceed the visible smoke limit and may also overload the engine. For these reasons, the negative adaptation must be adjusted to correspond to reduced charging pressures which brings the disadvantage that the maximum power of the engine cannot be utilized.
In a particular example of the above-described known rpm-based governor, there is provided a second control element which operates on the base of the turbo-charged pressure and is a stop in the controller which adjusts the maximum amount of fuel determined by the control cam on the basis of the induction tube pressure. The appropriate positions for the control elements are determined for the prevailing induction tube charging pressure and the associated rpm for full-load operation, possibly considering other limiting values, in only a single operational state. A charge pressure dependent correction of this type has the disadvantage that the increase of the fuel quantity during engine acceleration almost always occurs with a time delay because the fuel is being supplied on the basis of induction tube charging pressure which only increases when the exhaust gas turbo-charger is subjected to increased exhaust gas pressure and exhaust gas temperature. This interdependency of fuel quantity, charging pressure and exhaust gas pressure has a disadvantageous overall affect on the acceleration characteristics of an engine equipped with an exhaust-driven turbo-charger and using the known control system. In particular, the known charging pressure dependent correction cannot maintain engine operation below the permissible visible smoke limit in all operational domains and especially is it unable to do so in the non-stationary domain, i.e., when the engine is being accelerated from a condition of relatively low load, for in that case, the maximum fuel quantity for a given smoke limit depends very highly on the instantaneous engine speed as well as on the instantaneous turbo-charging pressure. It has been determined in experiments that the measure of the air mass delivered to the engine must be obtained from the absolute induction tube charging pressure and from measurements of the vacuum occurring during acceleration.
Measurements on engines have shown that the curves which plot the maximum fuel quantity as a function of rpm for every domain of turbo-charging pressure have different slopes so that the distances between the curves are different for a constant pressure difference, as is illustrated for example in FIG. 2 and will be discussed in more detail below.
A dependency of this nature, in which the maximum fuel depends on the instantaneous engine speed and the instantaneous absolute turbo-charging pressure (or the air mass) cannot be accounted for with the known control systems which have as their goal an adaptation of the maximum fuel quantity on either engine speed or induction tube charging pressure. Even when the signal related to induction tube charging pressure is superimposed on the rpm-related control process, such as is provided by the known and above-described centrifugal speed controller, the fuel quantity supplied is greatly in error and forces the designer to compromise in the manner of controlling the engine and furthermore will lead to excessive smoking and poor acceleration in substantial domains of the engine operation. An added difficulty in the use of the known controllers and governors for Diesel engines is the fact that the full-load limitation of the fuel quantity on the basis of engine speed and induction tube charging pressure does not depend only on engine speed and charging pressure but also on other variables, for example the thermal and mechanical strength of the engine and these additional considerations may force the designer to provide expensive constructions which at best provide only an approximation to the exact limiting fuel quantity.